Blending a fluorescent material to a plastic material to impart identification information has been studied. For example, a fluorescent material is blended in a plastic material used in a food tray, and by detecting fluorescence emitted from the tray, information can be read in the same way as a barcode or the like. Therefore, information on the production area of foods or the like can be imparted to the tray by blending a fluorescent material in a plastic material. In this case, it is necessary to irradiate the light of a specific wavelength in order to detect the fluorescence from the plastic material in which the fluorescent material is blended, but fluorescence spectra to be obtained varies depending on the kinds of fluorescent materials to be used, and therefore it is possible to impart information in cipher. The technology for imparting encrypted information with a fluorescent material by utilizing such a property has been receiving a lot of attention.
Plastic materials are in general fabricated by being melted at a high temperature (for example, about 300° C. for polycarbonate products), therefore a fluorescent material is required to be capable of emitting fluorescence with a sufficient intensity without decomposition even after the heating at a high temperature in the fabrication process. In the case where an inorganic fluorescent material such as Y2O3:Eu used in Braun tube for color TVs is used, there is no problem with heat resistance, but one problem thereof is that the inorganic fluorescent materials cannot be uniformly dispersed in plastic materials because the inorganic fluorescent materials are not dissolved in plastic materials. Moreover, another problem of inorganic fluorescent materials is that while the fluorescent emission can be observed in organic solvents, the fluorescence is not emitted when the inorganic fluorescent materials are blended in plastic materials.
On the other hand, it is known that organic rare-earth complexes in which an organic ligand is coordinated to a rare-earth ion can be uniformly dispersed in plastic materials and can emit fluorescence even when dispersed in plastic materials. However, up to now, it has tended to be difficult to blend organic rare-earth complexes in plastic materials because ligands constituting the complexes decompose when the complexes are heated to a temperature needed to fabricate plastic materials.
In such circumstances, it is shown in Patent Literature 1 listed below that a multinuclear rare-earth complex in which one or more types of molecules having a photosensitizing function are coordinated to a rare-earth ion has a high heat resistance and therefore can be blended in plastic materials and fabricated.